Proposal: generator#clone() and generator#goto()

Tab Atkins Jr. jackalmage at
Wed Aug 20 08:39:08 PDT 2014

On Wed, Aug 20, 2014 at 5:00 AM, Salvador de la Puente González
<salva at> wrote:
> Hello.
> Just a little presentation before the proposal. I'm Salvador de la Puente
> González (you can call me Salva, please). I'm working for Telefonica I+D in
> Mozilla Firefox OS and I'm a front-end developer, you can know more about me
> in and my github user is
> Now, these day I was thinking about a generator feature. I have a use case
> for a transpiler I'm developing: in the source language you have procedures
> much more like Linux processes. The language have a block sentence clone for
> cloning a procedure. After cloning, father procedure simply ignores the
> block while coned procedure execute sentences inside. Like an automatized
> |pid = fork() if (pid != 0) { .... }|.
> Now I'm addressing this problem from a AST perspective, transpiling
> procedures to a special generators able to fall through their sentences
> until reaching a specific yield.
> But I think it could be easier if I could do something like
> |generator#clone()| returning a new generator object. This new generator is
> paused of the same yield as the original one and have a new variable
> environment with a shallow copy of the original's variable environment. The
> variable environment's parent chain would be the same as the original one.
> Notice this is not related with data preservation as most of the objects
> being used by original generator will be shared with the new one (although
> this could be addressed as well) but with execution state.

Generators produce iterables, and you can just use a tee() function to
"clone" an iterable.  It's not hard to define yourself, and Python has
a reference implementation:

> In the same way, to provide a complete control over execution state inside
> generators I would like to propose |generator#goto(yieldLabel)| that could
> allow the developer to transfer the execution cursor to a labeled yield
> sentence. The result of the yield sentence could be the extended with the
> label for that yield.
> I know it does not suffice to be a cool feature but I think it could trigger
> very interesting uses cases and applications in backtracking or temper
> algorithms.

You can currently pass values back into the generator by passing an
argument to .next(), and the generator can use that to do whatever it
wishes, such as changing its internal state or jumping to a label.


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